Hydraulic piston machine

ABSTRACT

A hydraulic piston machine is proposed, with a cylinder body (1), which comprises at least one cylinder (2) with a piston (3) movable therein, and a control counter-plate (7) which engages the cylinder body (1) by way of a contact surface (6) and, on relative movement between the cylinder body and the control counter-plate parallel to the contact surface, connects the cylinder in dependence upon its position with inlet and outlet channels (11, 12) respectively, the control counter-plate (7) and/or the cylinder body (1) being provided, at least in the region of the contact surface (6, 9), with a friction-reducing layer (13). A machine of that kind should be inexpensive to manufacture yet should operate reliably even when the hydraulic fluid has no or only slight lubricating properties. For that purpose the friction-reducing layer (13) is in the form of an injection-moulded part.

The invention relates to a hydraulic piston machine with a cylinderbody, which comprises at least one cylinder with a piston movabletherein, and a control counter-plate which engages the cylinder body byway of a contact surface and, on relative movement between the cylinderbody and the control counter-plate parallel to the contact surface,connects the cylinder in dependence upon its position with inlet andoutlet channels respectively, the control counter-plate and/or thecylinder body being provided, at least in the region of the contactsurface, with a friction-reducing layer of plastics material.

In piston pumps, as known, for example from DE-AS 12 67 985, thecylinder body has to be pressed with a relatively strong pressureagainst the control counter-plate in order to keep the transition fromthe cylinders to the control counter-plate as well-sealed as possible.The larger are the unsealed areas in this region, the greater is theleakage, which has an adverse effect on the volumetric efficiency of themachine. Because of the high pressure, considerable frictional forcesact on the contact surface and, without supplementary measures, leadrelatively quickly to wear and tear or even to destruction of themachine. This phenomenon can be partially counteracted by lubricatingthe contact surface to reduce friction. The hydraulic fluid is used forthat purpose. This presupposes, however, that the hydraulic fluid hassatisfactory lubricating properties. This requirement considerablyrestricts the group of hydraulic fluids that can be used. Fluids whichhave satisfactory lubricating properties are in many cases harmful fromthe point of view of their impact on the environment, especially in sofar as synthetic oils are concerned.

For that reason, in a machine of the kind mentioned in the introduction,which is intended for use for water, it is known (JP 2-125 979) toarrange between the control counter-plate and the cylinder body aplastics material layer which is adhesively secured to the roughenedcontact surface. A number of manufacturing steps are required for this,however. It is also not always possible to ensure that the plasticsmaterial layer is reliably fixed to the cylinder body or to the controlcounter-plate. In particular, there is a danger that hydraulic fluidunder pressure will get between the plastics material layer and thecontrol counter-plate or cylinder body and detach the layer at leastpartially. This leads very rapidly to serious damage to the machine.

From DE 16 53 529 B2 it is furthermore known to provide a controlcontrol counter-plate disc with inserts of carbon, which are in the formof linear strips, to reduce the friction between the cylinder body andthe control counter-plate disc. Here, however, it is difficult to keepthe control counter-plate and the end face of the cylinder body closeenough to one another, so that considerable leakage can occur here.

The invention is therefore based on the problem of providing a hydraulicpiston machine that is inexpensive to manufacture and which can beoperated reliably even with hydraulic fluids that have few or nolubricating properties.

This problem is solved in a hydraulic piston machine of the kindmentioned in the introduction in that the friction-reducing layer is inthe form of an injection-moulded part.

Injection moulding on the one hand enables relatively thin layers to beachieved which can, on the other hand, be fixed securely to the partcarrying them. This simplifies manufacture quite considerably,especially when the layer is produced "in situ", that is, is injecteddirectly onto the cylinder body or the control counter-plate. Thefunction of "lubrication" is transferred to a machine element, namely tothe friction-reducing layer, which forms a surface layer in the regionof the contact surface. Combinations of materials are known which areable to slide on one another with relatively little friction even underrelatively large pressures. Through suitable selection of the materialfor the layer, taking into account the material of the controlcounter-plate or the cylinder body which rubs on this layer in theregion of the contact surface, friction in the region of the contactsurface can therefore be markedly reduced, without the need for alubricating fluid to be supplied. Since the layer is only a thininjection-moulded layer, however, the mechanical properties of themachine are otherwise unaffected, or not affected to an appreciableextent. The stability and thus the load rating in particular remainvirtually unchanged. The limitation to one layer also enables previouslyused components to be used virtually unchanged. Since thefriction-reducing material is applied only in the form of a layer, thereis practically no risk of the geometry of the cylinder body or thecontrol counter-plate undergoing changes because of increases ordecreases in temperature. Injection-coating or injection-mouldingproduces a connection of the plastics material with its correspondingcounter-part that is substantially more intimate than that produced byuse of adhesive. The plastics material of the injection-moulded layercan be selected in dependence upon the material which rubs against thefriction-reducing layer. Suitable combinations of materials enablecoefficients of friction that are entirely comparable with the values ofa fluid-lubricated contact surface or even exceed these to be achieved.Examples of plastics materials which may be considered for theinjection-moulded part are, in particular, materials from the group ofhigh-strength thermoplastic plastics materials on the basis of polyarylether ketones, in particular polyether ether ketones, polyamides,polyacetals, polyaryl ethers, polyethylene terephthalates, polyphenylenesulphides, polysulphones, polyether sulphones, polyether imides,polyamide imide, polyacrylates, phenol resins, such as novolak resins,or similar substances, and as fillers, use can be made of glass,graphite, polytetrafluoroethylene or carbon, in particular in fibreform. When using such materials, it is likewise possible to use water asthe hydraulic fluid.

Surface structures in the form of channels and other recesses arepreferably provided in the friction-reducing layer. Previously, thesesurface structures had to be produced by a mechanical machiningoperation on the corresponding contact surface, for example by millingor a different machining process. Now, however, the surface structurescan be incorporated directly in the friction-reducing layer, forexample, by producing them directly during moulding. For this purpose itis merely necessary for the moulding tool to have a correspondingnegative form in the region of the contact surface. Since a machiningstep, which is relatively time-consuming, can consequently be omitted,production costs can be considerably reduced. Moreover, it is likewisepossible to produce more precise structures, which could be producedonly with difficulty by secondary mechanical machining.

The friction-reducing layer is preferably provided on the controlcounter-plate, the control counter-plate having continuous openings andthe layer extending right through the continuous openings. The layer,which is of integral form in the region of the contact surface and inthe region of the continuous openings, now has, as it were, a holdingelement, which is able to accommodate forces that act parallel to thecontact surface, for example the frictional forces. Thefriction-reducing layer is accordingly held reliably in place on thecontrol counter-plate.

The cylinder body preferably has a pressure plate on the side facing thecontrol counter-plate, the pressure plate having for each cylinder acontinuous bore, which is connected to the cylinder by way of a bushthat is axially movable in the cylinder and/or the continuous bore, thepressure plate engaging the cylinder body by way of a compressionspring. The contact surface is therefore here formed between pressureplate and control counter-plate. Since the friction-reducing layerallows much higher pressure forces than previously to act on the contactsurface, this measure has the advantage that these pressure forces arekept away from the cylinder which is generally lined with a bushing. Theforce that presses the pressure plate and the control counter-plateagainst one another is essentially applied by way of the compressionspring. Because the bushes are able to move axially, even with the smallmovements that may occur it is possible to ensure that a connectionbetween the cylinder and the continuous bore is maintained. Frictionalforces which can act on the bush are largely accommodated by thepressure plate. It therefore becomes easier to balance out theco-operation between the pressure plate and control counter-plate, thatis, to effect an equilibrium between the forces produced by thecompression spring and the forces produced by relief of hydrostaticpressure at the contact surface.

Advantageously, provision is made for the part having thefriction-reducing layer to be surrounded on all sides, at least in thepressure region, by plastics material. There are then no gaps betweenthe part and the plastics material layer through which hydraulic fluidcould penetrate and cause damage. Perforations in the plastics materiallayer produced by fixing the corresponding part in the injection mouldcan be sealed subsequently. It is easier, however, to arrange theseperforations in a region not acted on by pressure. Simple wetting, thatis, pressureless wetting of the layer, cannot then lead to the hydraulicfluid penetrating between the layer and the part.

Advantageously, the control counter-plate has a low-pressure "kidney"which, at its end over which an opening passes first, has a groove whichproduces a directed jet at least immediately after the start ofcoincidence with the opening, wherein the distance between the start ofthe groove and a projected point of impact of the jet on a wall of thelow-pressure kidney is greater than 3.5 times the width of thelow-pressure kidney. When the opening, which may also be formed by thecontinuous bore, coincides with the low-pressure kidney, initially avery small opening appears which enlarges very quickly but at thebeginning discharges the hydraulic fluid still in the cylinder in a veryfine jet at great pressure. This is less inconvenient if the hydraulicfluid is an oil, because oil is relatively "soft". If water is beingused as hydraulic fluid, however, this jet, which is extraordinarilyfierce, results very rapidly in destruction of the plastics materiallayer and subsequently of the control counter-plate, because water isrelatively "hard", namely, about two to five times harder than oil. Toprevent such damage, the jet is directed so that it has to cover as longas possible a path within the hydraulic fluid. In this connection, it isfanned out and is braked so that a fierce jet of fluid no longer strikesthe wall. On the contrary, it is only a flow that is produced, which nolonger causes erosion.

The invention also relates to a method for manufacturing such a machine,in which the friction-reducing layer is produced by moulding, inparticular by injection-moulding. The friction-reducing layer can beapplied relatively thinly, but with the required accuracy, by themoulding or injection-moulding. If the layer is moulded directly ontothe part, a very durable bond is produced.

In this connection, the surface structures are preferably producedduring moulding through suitable shaping of the mould. Subsequentmechanical machining is largely avoided by this measure.

In this connection, it is especially preferred for the contact surfaceto be produced after moulding by surface grinding of protrudingprojections in the region of their ends. Although the geometry of thecontact surface can to a large extent be shaped with a high degree ofaccuracy by the moulding, it is easier to bring the ends of allprojections into one plane by surface-grinding after moulding than it isto shape the moulding tool so that the required accuracy is guaranteed.

The invention is described hereinafter with reference to preferredembodiments in conjunction with the drawing, in which

FIG. 1 shows a diagrammatic cross-section through a part of a pistonmachine according to a first embodiment,

FIG. 2 shows a detail view A corresponding to FIG. 1,

FIG. 3 shows a section III--III corresponding to FIG. 1, and

FIG. 4 shows a section according to a second embodiment.

A hydraulic machine has a cylinder body 1 in which at least one cylinder2 is arranged. A piston 3 is arranged to move up and down in thecylinder 2. The movement of the piston 3 is controlled by a slantingplate 4 against which the piston 3 lies via the intermediary of a slidershoe 5.

The opposite end of the cylinder body 1, that is, the side from whichthe piston 3 does not project, has a contact surface 6 with by means ofwhich the cylinder body 1 lies on a control counter-plate 7. If thecylinder body 1 is rotated in the direction of arrow 8, the contactsurface 6 of the cylinder body 1 slides over an opposing contact surface9 of the control counter-plate 7. On each rotation of the cylinder body1, an opening 10 in the cylinder coincides alternately with an intakebore 11 and a pressure bore 12 in the control counter-plate, which areconnected to an intake connection and a pressure connection,respectively, not illustrated more precisely. If the machine is used asa motor, the intake bore 11 is connected to a pressure connection,whilst the pressure bore 12 is connected to a tank connection. During anupward movement of the piston 3 in the cylinder 2 hydraulic fluid istherefore delivered into the cylinder 2, whereas during a downwardmovement the hydraulic fluid is expelled from the cylinder 2.

So that the hydraulic fluid uses only the assigned route, that is tosay, passes only through the intake bore 11 and the pressure bore 12, itis necessary for the remaining regions of the contact surfaces 6, 9 toseal off this fluid route. The cylinder body 1 and the controlcounter-plate 7 therefore have to engage one another under a certainpressure.

This pressure causes the frictional forces in the contact surfaces 6, 9to increase, however. Lubrication using fluid was previously used toreduce friction, with the hydraulic fluid acting as the lubricant.

So that hydraulic fluids which have no lubricating properties can alsobe used, the control counter-plate 7 is provided at least in the regionof the contact surface 9 with a friction-reducing layer 13. Thisfriction-reducing layer is formed from a plastics material, for examplenylon or another polyamide, polytetrafluoroethylene (PTFE) or polyarylether ketone, such as polyether ether ketone (PEEK). For that purposethe control counter-plate 7 has a core 14 which, as in previous controlcounter-plates, can be formed from metal. The core 14 is then coated inan injection-moulding process with the friction-reducing layer 13 insuch a way that the intake bore 11 and the pressure bore 12, which arealso in the form of continuous openings, are also lined with the layer13. This results in the layer 13 being provided integrally with holdingelements 15 which project at right angles to the contact surface 9 andare able to accommodate forces that act approximately parallel to thecontact surface 9. Because the core 14 is completely encased by thefriction-reducing layer 13, there are no gaps through which hydraulicfluid could penetrate between the core 14 and the layer 13. The controlcounter-plate 7 can therefore also be used at high pressures.Perforations in the layer 13, which can result from fixing the core 14in an injection mould, are arranged so that they can be wetted only bypressureless hydraulic fluid.

Surface structures in the form of recesses 16 and projections 17 areworked in the contact surface 9. These surface structures 16, 17 can becreated during moulding around the core 14 if the mould hascorresponding negative shaping. Subsequent machining of the controlcounter-plate 7 can then largely be dispensed with. It is merelynecessary to bring all the outwardly projecting ends of the projections17 together into one plane. This can be effected, for example, bysurface grinding.

The friction-reducing layer may, of course, be equally well provided onthe contact surface 6 of the cylinder body 1. In that case, provisioncan also be made for it to enclose the cylinder body 1 completely.

As is apparent from FIG. 3, the intake connection 11, which is here inthe form of a low-pressure kidney, has a groove 23 at its commencement.If the cylinder body 1 is rotated in a clockwise direction, the opening10 of the cylinder 2 coincides with the groove 23 of the low-pressurekidney 11. Because there is always a certain residual pressure from thehigh pressure side still in the cylinder 2, the hydraulic fluid escapesat that instant through the gap formed between the groove 23 and theopening 10 at a relatively high pressure in a powerful jet 24. Tomoderate the effects of this jet, the jet 24 is directed, which can beachieved relatively easily by suitable shaping of the groove 23. The jet24 is hereby directed so that it does not meet the wall of thelow-pressure kidney 11 until it has covered a path that is at least 3.5times the width of the low-pressure kidney 11. The jet 24, or moreaccurately, the fluid moved in it, therefore has to cover a relativelylong route within the hydraulic fluid, during which it is braked by thesurrounding hydraulic fluid. The jet 24 is fanned out and thus loses itsintensity.

FIG. 4 shows a further construction, in which the cylinder body 1additionally has a pressure plate 18 which bears against the cylinderbody 1 by way of a compression spring 19. The pressure plate 18 hascontinuous bores 20 which can be caused to coincide with the intake bore11 and the pressure bore 12 respectively as the cylinder body 1 rotates.Bushes 21 which are mounted so as to be axially displaceable in thecylinder 2 and in the pressure plate 18 are let into the pressure plate18. The bushes 21 guarantee a reliable, that is to say, tightly sealedfluid connection even when the cylinder body 1 and the pressure plate 18move axially relative to one another. Such a movement is possible byvirtue of the compression spring 19. It is, however, of only smallextent.

In this construction, the pressure plate 18 is sheathed with afriction-reducing layer which is also led through the continuous bore20. The pressure plate 18 is therefore completely surrounded by theplastics material forming the friction-reducing layer 22.

As is apparent, the friction-reducing layer 13, 22 is very thin. Afterthe control counter-plate 7 and the pressure plate 18 have been coated,these two parts have approximately the same thickness as before. Theyare therefore by and large mechanically just as strong as a part madeexclusively of metal. On account of the friction-reducing layer,however, the cylinder body 1 and the control counter-plate 7 can bepressed against one another harder, that is to say, with greater force,than previously, with the result that leakage is reduced and efficiencyis increased without the mechanical performance being adversely affectedby higher frictional forces.

Coating of the control counter-plate 7 produces the advantage thathydraulic fluid is unable to penetrate into the gaps between the layer13 and the core 14, which could destroy the plastics material. Becausethe continuous bores 11, 12 also are lined with the plastics material,the plastics material can be regarded here as a tubular connection whichguides the hydraulic fluid from a stationary part, namely, the controlcounter-plate 7, to a rotating part, namely the cylinder body 1.

The present embodiment is illustrated as an axial piston machine, whichcan be used both as a motor and as a pump. It is, however, equallypossible for the friction-reducing layer to be incorporated incorresponding parts of a radial piston machine.

We claim:
 1. A hydraulic piston machine having a cylinder body and atleast one cylinder with a piston movable therein, a controlcounter-plate which engages the cylinder body by way of a contactsurface and, on relative movement between the cylinder body and thecontrol counter-plate parallel to the contact surface, connects thecylinder in dependence upon its position with inlet and outlet channelsrespectively, at least one of the control counter-plate and the cylinderbody being provided, at least proximate the contact surface, with afriction-reducing layer of plastic material, and in which thefriction-reducing layer is in the form of an injection-moulded part. 2.A machine according to claim 1, in which surface structures in the formof channels and recesses are provided in the friction-reducing layer. 3.A machine according to claim 2, in which the friction-reducing layer isprovided on the control counter-plate, the control counter-plate havingcontinuous openings and the layer extending through the continuousopenings.
 4. A machine according to claim 1, in which the cylinder bodyhas a pressure plate on a side facing the control counter-plate, thepressure plate having for each cylinder a continuous bore which isconnected to the cylinder by way of a bush that is axially movable in atleast one of the cylinder and the continuous bore, the pressure plateengaging the cylinder body via the intermediary of a compression spring.5. A machine according to claim 1 in which one of said controlcounter-plate and said cylinder body has the friction-reducing layersurrounded on all sides, at least in a pressure region, by plasticmaterial.
 6. A machine according to claim 1, in which the controlcounter-plate has a low-pressure kidney which, at its end over which anopening passes first, has a groove which produces a directed jet atleast immediately after the start of coincidence with the opening,wherein the distance between the start of the groove and a projectedpoint of impact of the jet on a wall of the low-pressure kidney isgreater than 3.5 times the width of the low-pressure kidney.
 7. A methodfor manufacturing the hydraulic machine defined by claim 2, in which thesurface structures are produced during moulding through suitable shapingof the mould.
 8. A method for manufacturing the hydraulic machinedefined by claim 7, in which the contact surface is produced aftermoulding by surface grinding of protruding projections in the region ofends of the projections.